Fiberglass Fillers Guide: Aerosil, Q Cell, Fairing & Structural Fillers Explained
What Are Fiberglass Fillers?
Fiberglass fillers are powders, fibers, or pre-mixed compounds used with polyester resin or epoxy resin to create custom mixtures for marine, repair, and composite applications.
They are commonly used in:
- Fiberglass boat repair
- Marine fairing and surface preparation
- Composite construction
- Surfboard building
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Types of Fiberglass Fillers (What to Use and When)
1. Lightweight / Fairing Fillers (Easy Sanding)
These fillers are designed to create smooth, sandable surfaces and are commonly used before primer or paint.
- Q Cell – Lightweight fairing filler, sands easily
- USC Lightweight Filler (15310) – Easy sanding repair filler
- Evercoat Glazing Putty – Final finishing and pinhole filling
- Fasco Fas Fair – Lightweight fairing compound
- Fairing Putty – General smoothing applications
Best for: Fairing boat hulls, smoothing fiberglass, surface prep
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2. Professional Marine Fairing Compounds
These are high-end systems designed for marine coatings like Awlgrip.
Best for: Boat hull fairing, Awlgrip systems, professional marine finishing
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3. Structural / Strength Fillers
These fillers increase strength and control resin thickness for structural applications.
- Aerosil (fumed silica filler) – High-strength thickener used to prevent sagging and improve bonding strength
- Chopped Strand (1/4") – Reinforces resin with fiberglass strands
- Milled Fibers (1/32") – Adds high structural strength
Aerosil is often referred to as a Cabosil-type filler, a commonly searched term for fumed silica used to thicken polyester and epoxy resin.
Best for: Structural repairs, bonding, thickening resin, vertical surfaces
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4. Bonding & Adhesive Fillers
These are used for structural bonding and assembly.
Best for: Bonding fiberglass, core materials, and structural components
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5. Heavy-Duty / Body Fillers
Pre-mixed fillers used for quick repairs and thick applications.
Best for: Automotive fiberglass repair, thick fills, fast patching
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Which Fiberglass Filler Should You Use?
| Application | Recommended Filler |
|---|---|
| Smoothing / fairing | Q Cell, glazing putty, fairing putty |
| Boat hull fairing | Awlfair, fairing putty |
| Strength / reinforcement | Aerosil, chopped strand, milled fibers |
| Bonding materials | Adtech, bonding putty |
| Quick repair | Duraglass |
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How to Mix Fillers with Resin
- Start with your base resin (polyester resin or epoxy)
- Gradually add filler while mixing
- Adjust thickness (often described as “peanut butter consistency”)
- Add catalyst (MEKP)
- Apply immediately
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Polyester vs Epoxy Fillers
Most fillers work with both systems, but performance depends on the resin. Polyester systems are more cost-effective, while epoxy resin provides better strength and adhesion.
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Common Mistakes to Avoid
- Using the wrong filler for the job
- Adding too much filler (reduces strength)
- Not mixing thoroughly
- Skipping proper surface prep
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Frequently Asked Questions
What is Aerosil used for?
Aerosil is used to thicken resin, prevent sagging, and improve bonding strength in fiberglass and composite applications.
Is Aerosil the same as Cabosil?
Aerosil and Cabosil are both fumed silica fillers used to thicken resin and increase strength. Cabosil is a commonly searched trade name, while Aerosil performs the same function.
What is Q Cell used for?
Q Cell is used for fairing and smoothing fiberglass surfaces because it sands easily.
What is the best filler for boat repair?
For fairing, use Q Cell or Awlfair. For structural strength, use Aerosil or fiberglass fibers.
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